Paulgrothite
Chemical formula: Cu<sup>2+</sup><sub>9</sub>Fe<sup>3+</sup>O<sub>4</sub>(PO<sub>4</sub>)<sub>4</sub>Cl
Paulgrothite is an extremely rare copper and iron phosphate of dark green color, discovered in the products of fumaroles of the Tolbachik volcano in Russia.
Description
## Characteristics Paulgrothite is a mineral from the averievite group, belonging to the phosphates. It forms small, tabular or flattened crystals, which usually occur as dense aggregates and coatings. Its characteristic feature is a dark green to greenish-black color. In thin fragments, it becomes translucent, revealing an intensely green color. ## Physical Properties Its hardness on the Mohs scale is approximately 4. It exhibits a vitreous, in places slightly greasy luster. The density calculated based on the chemical formula and unit cell parameters is 4.20 g/cm³. The mineral does not show fluorescence under ultraviolet light. ## Colors and Varieties This mineral occurs in uniform colors, from dark, emerald green to an almost black shade. No colored or commercial varieties have been distinguished. ## History and Name The mineral's name honors Paul Heinrich Ritter von Groth (1843–1927), an eminent German mineralogist and crystallographer, professor at the University of Munich and author of the fundamental work "Chemische Krystallographie". Paulgrothite was officially approved by the International Mineralogical Association (IMA) in 2021 (IMA2021-085). Its discovery and description were made by a team of mineralogists led by Igor V. Pekov in 2022 at the Tolbachik volcano.
Diagnostic features
## Identification Identifying paulgrothite based on visual characteristics is very difficult. Key clues are its specific occurrence environment (volcanic fumaroles), dark green color, tabular crystal habit, and co-occurrence with other rare minerals from Tolbachik. It is a typical "micromount mineral". ## Distinguishing from Similar Minerals It can be easily confused with many other dark green or black minerals occurring in the same environment, such as averievite, arsmirandite, lammerite, or tenorite. Definitive distinction is possible only through advanced analytical methods, such as X-ray diffraction (XRD) and chemical microanalysis (EDS). ## Crystal Forms It forms tabular, hexagonal crystals not exceeding 0.2 mm in size. These crystals often grow together into dense, rosette-like aggregates, crystal brushes, and crusty coatings on the rock surface.
Geological environment
## Genesis Paulgrothite is a mineral of volcanic origin, crystallizing in high-temperature (450–600°C) exhalative fumarole environments. It forms as a result of the reaction of volcanic gases rich in chlorine, phosphorus, and metals with atmospheric oxygen, through the process of sublimation on the surface of porous basaltic lava. ## Mineral Associations This mineral occurs in association with many other rare fumarole products. It most commonly co-occurs with hematite, tenorite, averievite, alarsite, andradite, arsmirandite, johillerite, lammerite, and nabokoite. ## Localities The only confirmed locality of paulgrothite in the world is its type locality - Arsenatnaya fumarole, located on the second cinder cone of the Northern Breakthrough of the Great Fissure Eruption of Tolbachik volcano on the Kamchatka Peninsula in Russia.
Rarity
Extremely rare
Collector aspects
## Quality Criteria For micromineral collectors, the value of a paulgrothite specimen depends on several factors. The most important is the abundance of the mineral on the rock matrix and the quality of crystal development – the sharper and better defined, the more valuable the specimen. Rich associations with other rare and aesthetic minerals from this locality are also highly prized. ## Popular Localities Paulgrothite specimens come exclusively from one place in the world – the fumarole of Tolbachik volcano in Kamchatka. This is a classic locality for collectors specializing in rare minerals and products of volcanic activity.
Care and storage
## Cleaning As an extremely rare and likely sensitive mineral, it should not be cleaned mechanically or chemically. For dust removal, at most a gentle stream of compressed air from a safe distance can be used. ## What to Avoid Contact with water, acids, and other chemicals must be strictly avoided. As a chloride, it may be hygroscopic and sensitive to moisture in the air. It should be protected from sudden temperature changes and direct sunlight. ## Storage Paulgrothite specimens are best stored in tightly sealed, dry containers, e.g., "micromount" type, which protect against moisture, dust, and mechanical damage. Storage in stable room conditions is recommended.